Allogeneic bone marrow transplantation (BMT) is currently used for the treatment of a variety of cancers but graft-versus host disease (GVHD), immune deficiency following the transplant, and relapse remain significant obstacles limiting efficacy. Development of a means to improve the anti-tumor effects of BMT is of considerable importance. Natural killer (NK) cells have been shown to mediate numerous anti-tumor effects both in vivo and in vitro. We have previously demonstrated that donor-type NK cells can prevent GVHD and promote anti-tumor effects following allogeneic BMT in mice. NK cells can be regulated, both in positive and negative manners by a variety of mediators. This proposal will develop means to optimize NK cell recovery and activity following allogeneic BMT resulting in greater anti-tumor effects. To do this three specific aims are proposed: Specific Aim 1 will build on our recent data demonstrating that regulatory T (Tregs) cells can suppress NK activity. We propose to determine the mechanism(s) underlying this suppression and to ascertain the effects of Treg depletion on NK reconstitution and activity following allogeneic BMT. Specific Aim 2 will then seek to accelerate donor NK cell recovery post-BMT through the use of hydrodynamic gene delivery of IL15. Effects on GVHD, donor reconstitution as well as anti-tumor effects will be determined. Specific Aim 3 will determine the role of NK cell subpopulations on donor recovery, GVHD protection, and anti-tumor effects by building on our data that the interactions between these subsets result in significant effects on activity in vivo. The data obtained from this proposal should yield significant insights into NK cell biology as well as allowing the development of approaches that result in superior anti-tumor effects.